Motors for driving used in electric vehicles and hybrid vehicles are required to provide significant power output so that permanent magnet motors including a rare earth element that retains intense energy are generally used. The motors for driving use, from among such permanent magnet motors, embedded-type magnet motors, which can satisfy the requirement to provide a large torque at low speeds and a wide rotation speed range.
Torque fluctuations of a motor are causes of noises and vibrations. In particular, in the case of electric vehicles, there arises the problem that the torque fluctuations make the ride uncomfortable at low speeds. Conventional motors generally adopt a countermeasure to provide skew in order to reduce the torque fluctuations. For example, there is known a motor in which an electromagnetic steel sheet provided with grooves is arranged on the side of outer periphery of a magnet embedded in a rotor and the grooves are arranged as being displaced in a direction along the periphery of the rotary shaft one portion from another.